Sexual imprinting is an early learning process by which young birds acquire the characteristics of a potential sexual partner. The physiological basis of this learning process is an irreversible reduction of dendritic spines in two forebrain areas, the LNM (lateral nido-mesopallium) and the MNM (medial nido-mesopallium). The aim of the present study was to investigate whether these two brain areas are activated if the imprinted stimulus is presented to the adult bird after the end of the sensitive period. One group of zebra finch males was reared by their own parents. These birds, as adults, showed an exclusive preference for their own species in choice tests between a zebra finch and a Bengalese finch female. If exposed as adults to a zebra finch female, LNM and MNM showed lower activation, as measured by ZENK expression, compared to males exposed to a Bengalese finch female. A second group was reared by Bengalese finches and was exposed at day 100 to a zebra finch female for 1 week. As shown earlier, this regime leads to mixed choices, the birds are courting Bengalese and zebra finch females with a fixed ratio (preference score). If these birds were exposed to a zebra finch female as adults, the ZENK expression within LNM was much higher compared to group 1, and it showed a strong tendency to correlate negatively with the preference score: Birds with higher zebra finch preference showed lower activation compared to those with a low zebra finch and a high Bengalese finch preference. We propose that higher ZENK activation in group 2 is due to the rearing by a foster species which may result in a more complex neuronal network. The negative relation between activation and preference score may be explained by special properties of the LNM and MNM networks. (c) 2006 Elsevier B.V. All rights reserved.